April 2011

Power and water

Alternatives to chemical batteries in transportation

Fuel cells have the advantage of being non-polluting, non-combusting and producing no dangerous emissions, only water and heat. How close to broad-scale commercialization is this technology? What are the major opportunities and challenges?

Concentrated, highly efficient fuel

Hydrogen is clean: it can be extracted from fossil fuels, a process that creates only a small amount of pollution, or made pollution-free with renewable fuels, in particular solar or wind. Filling stations that dispense hydrogen made with solar are already in operation on all continents.

Many car-makers have engaged in fuel cell projects, using fuel cells primarily to recharge lithium-ion batteries. But despite obvious advances – for individual transportation – fuel cell technology needs to still overcome a number of hurdles before it can reach broad-scale commercialization.

For one, since hydrogen is a very light gas, it is difficult to store a large amount in a small space. This is a challenge for automobile engineers, who need to match the 500-kilometre range of conventional automobiles, while keeping cost, vehicle weight and volume for hydrogen storage down.

Urban transport

Urban bus systems are a logical application for fuel cells, largely because refuelling is concentrated at fleet depots. Buses can also store larger volumes of fuel than other vehicles. Government support for the testing of hydrogen vehicles is significant. According to the London Hydrogen Partnership, annual investment by automotive companies and governments around the world in hydrogen vehicle technologies exceeds USD 3 billion. In the EU (European Union) hydrogen-fuelled busses already operate as part of public transport systems in 10 cities.

The Daimler Chrysler Citaro fuel cell bus, one of the makes used in the CUTE (Clean Urban Transport for Europe) project, has a range of 200 to 300 km, carries 60 to 70 passengers and is powered by a 250kW PEM (proton exchange membrane) fuel cell (200kW shaft power). It has up to 44 kg of compressed hydrogen gas stored at 350 bar.

London first trialled CUTE fuel cell buses between 2003 and 2007. On 26 January 2011 it introduced a fleet of eight hydrogen buses that link Tower Gateway Docklands Light Railway station with Covent Garden. But the CUTE project is now global in scope with buses being trialled in 10 cities on three continents.

Olympic Games and fuel cells

Major sporting events are increasingly being seen as an opportunity for governments to roll out infrastructure, with a view to encouraging manufacturers to commit to work towards volume manufacturing and associated cost reductions. Hydrogen and fuel cell transport has become a regular feature at the Olympic Games.

First in Turin, Italy, during the 2006 Winter Olympics, and then at the 2008 Summer Olympics in Beijing, China, where 20 fuel cell cars and three fuel cell buses were demonstrated.

The Canadian province of British Columbia used the 2010 Vancouver Winter Olympic and Paralympic games to roll out its “Hydrogen Highway”:seven fuelling stations between Vancouver Airport and the resort town of Whistler. The world’s largest single fuel cell fleet, some 20 buses, went into operation for the Canadian games. BC Transit’s new Whistler Transit Centre, which includes the world’s largest hydrogen-vehicle fuelling station, was officially opened at the games.

Beijing already had a hydrogen refuelling station, with an on-site methane reforming facility, ahead of the 2008 games. Beijing had been selected, alongside Shanghai, by the GEF (Global Environment Facility) of the World Bank for the Fuel Cell Bus Demonstration Project. In 2007, an Anting hydrogen refuelling station was opened in Shanghai. Subsequently, at the 2010 Shanghai World Expo, 196 fuel cell vehicles, including six fuel cell buses, were deployed.

Beijing and Shanghai are both involved in the Chinese Government’s 1,000+ Green Vehicles in each City’ programme. The programme, which was launched in 13 Chinese cities, provides subsidies for the purchase of hybrid, electric and fuel cell vehicles.

The American state of California is, unsurprisingly, home to a number of fuel cell bus trials. The SunLine Transit Agency fleet in California uses UTC (United Technologies Company) Power fuel cells. And three AC Transit (Californian Alameda-Contra Costa Transit District) buses also run on UTC Power fuel cell systems.

Brazil, which will host both the 2014 FIFA (the International Federation of Association Football) World Cup and the 2016 Olympic Games, has invested significantly in fuel cell buses over the years and we can expect some on display during the forthcoming sporting events. A Brazilian hydrogen fuel cell prototype bus began operation in São Paulo in 2009 as part of the Ồnibus Brasileiro a Hidrogênio programme.

Fuel cells in the air

Ground transportation may not be the only place where fuel cells could have an impact. PEMFC (polymer electrolyte membrane fuel cell) fuel cells were first tested in a commercial airplane in 2008. The test flight was carried out on an A320 aircraft owned by the DLR (Deutsches Zentrum für Luft- und Raumfahrt e.V., the German Aerospace Centre). It was powered only by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane, as it was called, used a PEM fuel cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller. During the test, the fuel cell system produced up to 20KW of electrical power. It powered the electric motor pump for the aircraft’s back-up hydraulic circuit and controlled the spoilers, ailerons and elevator actuator.

"Currently fuel cell systems for commercial aviation are at an early stage of research and technology, and today it is not foreseeable that they would be used for commercial aircraft propulsion,” says Airbus. “This requires a thousand times the electric energy that was produced during the test flight."

"To use fuel cells more extensively on board commercial aircraft, further improvements need to be made in terms of the amount of energy they produce versus their weight (ratio KW per kg)," Airbus says. Airbus believes that fuel cells could eventually replace the APU (Auxiliary Power Unit) that at present fuels aircraft functions such as starting up the main engine and the air conditioning, which would provide emission-free ground operations.

Other uses of electricity for public transport

The March 2010 e-tech looked at the first use of fuel cells in trains in providing emergency power and changing the present infrastructure, but particularly as a replacement of the propulsion system of diesel locomotives to help reduce CO2 (carbon dioxide) emissions.

Technology challenges of fuel cells

Fuel cells could be the answer to many problems, but they also comprise some major challenges, some of which are particularly difficult to overcome and may represent a major hurdle for individual transportation systems. That explains why most progress seems to occur in public transportation systems. Challenges include cost, durability, size, weight, thermal and water management and infrastructure.

Cost

Many components of fuel cells are expensive. For PEMFC systems, proton exchange membranes, precious metal catalysts (usually platinum), gas diffusion layers and bipolar plates make up 70 % of the cost. Many companies are working on techniques to reduce cost in a variety of ways, including reducing the amount of platinum needed in each individual cell. Experiments include catalysts enhanced with carbon silk, which allows a 30 % reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance. Monash University in Melbourne, Australia, uses PEDOT (Poly (3,4-ehtylenedioxythiophene) as a cathode. A 2011 published study documented the first-ever metal-free electro catalyst using relatively inexpensive doped carbon nanotubes that are less than 1 % the cost of platinum and are equal or superior in performance. While promising, all these technologies are still far from commercialization.

Water and air management

In PEMFCs, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Research focuses on methods to control the flow of water in the cells. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.

Temperature management and durability

The same temperature must be maintained throughout the cell in order to prevent its destruction. This is particularly challenging since a lot of heat is constantly generated as a direct result of the chemical reaction within the fuel cell.

Currently PEMFC membranes tend to degrade while fuel cells cycle on and off, particularly as operating temperatures rise. Since cars stop and start frequently, it is important for membranes to remain stable under cycling conditions. A particular challenge to durability is large differences in operating temperatures.

Safety and delivery

A kilo of hydrogen has three times the energy of a kilo of gasoline, and because of its high energy content, hydrogen needs careful handling.

Infrastructure

Last but not least, in order for PEMFC vehicles to become a viable alternative for consumers, a hydrogen generation and delivery infrastructure must be put in place.

Fuel cells

The principle of the fuel cell was developed in 1839 by Sir William R. Grove. He discovered that hydrogen and oxygen could be combined to produce water and an electric current. It wasn't until 1932 that Francis T. Bacon produced the first successful fuel cell, a device using a hydrogen-oxygen cell with alkaline electrolytes and nickel electrodes. This was later used by NASA (the United States’ National Aeronautics and Space Administration) to power Apollo and Gemini space explorations and produce drinking water for the crews. Today, fuel cell technology is used in cars, buses, office buildings and homes, phones and laptop computers and everything in between. Fuel cell systems can be extremely efficient over a large range of sizes (from 1 kW to hundreds of megawatts). Some systems can achieve overall efficiencies of 80 % or more when heat production is combined with power generation.

The differentiating element in the various sorts of fuel cells is the electrolyte, which is also what defines the operating temperature of the fuel cell. The solid oxide technology produces temperatures that can reach 800 degrees Celsius, which is fine for home use where the additional heat can be recycled and used in other ways, but not ideal for a moving vehicle. The best method for public transport and utility vehicles is the PEMFC (polymer electrolyte membrane fuel cell) technology that uses a permeable polymer membrane work to exchange protons at a temperature of around 80 degrees Celsius.

London's fuel cell bus. Source: European Hydrogen Association.

Fuel cell in Airbus. Credit: Airbus Deutschland.

HyPM-16 fuel cell power.

IEC TC 105

IEC TC (Technical Committee) 105: Fuel cell technologies, has already prepared a number of standards on safety, installation and performance of both stationary fuel cell systems and for transportation both for propulsion and as APUs. The TC has been very active since its first plenary meeting in 2000 in Frankfurt, Germany.

TC 105 Secretary Wolfgang Winkler is a mechanical engineer specializing in fuel cells system thermodynamics and system integration.

The market for fuel cells is expected to grow to an estimated USD 10 billion by 2020 with more than 90 % of that encompassing North America, the Asia-Pacific region and the European Union. The size reflects the demand and application for energy conversion systems, starting from micro devices and proceeding to farm equipment, cars, boats and trains to larger stationary systems.

Winkler says that International Standards can help to open the market by showing potential investors that groundwork has already been well prepared and investigated, thus reducing risk to investors so they can indirectly mobilize capital.

The transportation sector is an interesting area for TC 105. Standardization in automotive applications is carried out jointly with ISO (International Organization for Standardization) TC 22/SC (Subcommittee) 21: Electrically propelled road vehicles.

Besides the automotive mass market, TC 105 is focusing on a number of other transportation applications.

The fuel to be used can be a logistic one, standardized worldwide, or specific in special cartridges as hydrogen or methanol.

In future, the fuel cell and battery combination is expected to be used for a hybrid mix of fuel cells and electric storage for batteries and super capacitors. Already under development is a fuel cell and battery hybrid that helps recover energy as forklifts raise and lower loads.

With good cooperation between battery and fuel cell, energy has to be supplied as the forklift raises its load and then recovered as the lift comes back down. The recovered electric power can be used in a generator to charge the battery and keep the cycle going.

In cars the same principle is applied with different movements in two directions, for example up a hill and then down. "It's like a pendulum," explains Winkler. “During acceleration, or going uphill, energy of a battery is consumed. But during slowing down or going downhill the energy can be partially recovered and stored again in the battery. The fuel cell is needed to replace the energy losses caused mainly by friction."